1. Field of the Invention
The present invention relates to an optical transmission line wherein signal light is relayed by optical amplifiers linked together.
In the past, for long distance optical communications, the general practice was to perform optical-electrical conversion at predetermined intervals so as to process the signals as electrical signals to reproduce them, then perform electrical-optical conversion and once again send the converted signals on to the next regenerative repeater.
In recent years, however, optical amplifiers have been able to be realized at low cost, so development is being made of commercial optical transmission lines with multistage optical AMP's connected in cascade instead of the regenerative repeaters. Optical amplifiers are simple in circuit construction compared with the conventional regenerative repeaters, so are higher in reliability and further are lower in costs, so are particularly suitable when used for long distance ocean optical transmission lines.
2. Description of the Related Art
In general, optical transmission lines relayed with multistage optical amplifiers, as explained in detail later, are comprised of an optical transmission line (optical fiber) and a plurality of optical amplifiers connected in cascade along the optical transmission line. Signal light is transmitted from the input end of the optical transmission line and is relayed successively by the optical amplifiers until it reaches the output terminal of the optical transmission line. The input end is provided with an optical transmitter, while the output terminal side is provided with an optical receiver.
In such an optical transmission line relayed with optical amplifiers linked together, each time light is relayed and amplified by the different stages of the optical amplifiers, the amplified spontaneous emission (ASE) increases monotonously. Along with this monotonous increase in the amount of ASE, the nonlinear effects of phase noise, which is caused by the Kerr effect, increase. The nonlinear effects are proportional to the optical power of the amount of ASE of the output signal light from the output terminal of the optical transmission line, so the nonlinear effects increase proportionally with the integral value of the amount of ASE along with distance.
The easiest way to reduce the integral value of the amount of ASE along with distance is to reduce the bandwidths of the optical filters provided in the different stages of optical amplifiers. However, if the bandwidths of the optical filters are reduced across the board, there is the new problem that wave distortion occurs in the output signal light. This is because if the bandwidths of the optical filters are reduced across the board, there is a large frequency band limitation in the signal light spectrum.